This invention relates to optical fiber communication systems and, in particular, to reconfigurable add/drop devices for adding or dropping communication channels at intermediate points within such systems.
Optical fiber communication systems are beginning to achieve their great potential for the rapid transmission of vast amounts of information. In essence, an optical fiber system comprises a light source, a modulator for impressing information on the light, an optical fiber transmission line for carrying the optical signals and a receiver for detecting the signals and demodulating the information they carry. Increasingly the optical signals are wavelength division multiplexed signals (WDM signals) comprising a plurality of distinct wavelength signal channels.
Add/drop devices are important components of WDM fiber communication systems. Such devices are typically disposed at various intermediate points along the transmission fiber (called nodes) to permit adding or dropping of signal channels at the nodes. Thus, for illustration, an add/drop device would permit a transmission line from New York to Los Angeles to drop off at Chicago signal channels intended for Chicago and to add at Chicago signal channels intended for New York and Los Angeles. As the number of nodes increases, the number of add/drop devices increases, and their cost and effect on the system becomes appreciable.
A straightforward approach to adding or dropping channels in a system carrying a number of channels (say N) in a single fiber is to first separate the fiber transmission signal into its N component signal channels, to redirect the N channels to desired pathsxe2x80x94some of which continue on the main transmission path and some of which are dropped. The channels which are to be continued and those that are to be added are then recombined into a single transmission signal and transmitted to the next node.
This approach is conventionally implemented by a 1xc3x97N demultiplexer followed by a switch array and then an Nxc3x971 multiplexer. The demultiplexer receives the transmission signal and separates it into its N component signal channels. The switch appropriately directs each of the N channels, and the multiplexer combines the continued and added channels for transmission.
The difficulty with this conventional approach is that it is needlessly expensive and deleterious to the quality of the transmitted signals. The demultiplexer/multiplexer pair becomes expensive as the total number of channels (N) increases and the wavelength spacing between channels decreases. Numerous closely spaced channels require more and higher precision components. And the devices introduce loss in the signal channels in amounts which may vary from channel to channel.
Accordingly there is a need for an improved, add/drop device for optical fiber communication systems.
In accordance with the invention, an N channel WDM system is provided with add/drop capability by distributing the N channels among M paths where N greater than M, filtering each of the M paths through P add/drop filters where Mxc3x97P=N, and recombining the M filtered paths for transmission. This can be implemented with an Mxc3x971 demultiplexer, tunable Bragg grating filters and a 1xc3x97M multiplexer.